The present invention relates generally to millimeter and submillimeter wave amplifiers and more particularly, to gyrotron traveling-wave amplifiers for wide-band operation at high power levels.
The power levels available from coherent electromagnetic sources at millimeter wavelengths have been revolutionized by the advent of the gyrotron oscillator. However, information carrying systems such as radar and communications require an amplifier mechanism with substantial instantaneous bandwidth rather than simply an oscillation mechanism. Fast wave gyrotron traveling wave amplifiers currently proposed, have demonstrated power levels (34 kw) that are an order of magnitude greater than those available in conventional millimeter wave traveling-wave take amplifiers. However, their bandwidth is severely limited (approximately 1.5% bandwidth). An alternate type of gyrotron amplifier is the slow wave amplifier. However, its performance is severely limited by dielectric breakdowns, by limited heat dissipation capacity, and by degradation in performance due to the velocity spread of the electron beam. It is the intent of the present invention to ameliorate these difficulties.
As a preliminary to the discussion of the preferred embodiment, a general discussion of the operation of a gyrotron will be provided.
The gyrotron is a new type of microwave device employing the electron cyclotrons maser mechanism. It ideally consists of an ensemble of monoenergetic electrons following helical trajectories around the lines of an axial magnetic field inside a fast wave structure such as a metallic tube or waveguide. The physical mechanism responsible for the radiation in the gyrotrons has its origin in a relativistic effect. Initially, the phases of the electrons in their cyclotron orbits are random, but phase bunching (relativistic azimuthal bunching) can occur because of the dependence of the electron cyclotron frequency on the relativistic electron mass (.OMEGA..sub.c =eB/.nu.mc). Those electrons that lose energy to the wave become lighter, rotate faster, and, hence, accumulate phase lead, while those electrons that gain energy from the wave become heavier, rotate slower, and accumulate phase lag. This rotating electron interaction with the wave results in phase bunching such that the electrons radiate coherently and amplify the wave. Energy transfer from the electrons to the wave is optimized when .omega.-k.sub.z .nu..sub.zo -s.OMEGA..sub.c .gtoreq.0, where .omega.,k.sub.z,v.sub.zo, s, and .OMEGA..sub.c, are, respectively, the wave frequency, axial wave number, axial electron velocity, cyclotron harmonic number, and electron cyclotron frequency.
In essence, there is an intrinsic preference for relativistic azimuthal phase bunching in the presence of an electromagnetic wave. This bunching yields a different configuration of electrons in a lower energy state. If the incident wave has a frequency slightly larger than .OMEGA..sub.c or its harmonics, then stimulated emission will occur. Since this bunching mechanism occurs in phase with the electromagnetic wave, the stimulated radiation emission from the bunching is also emitted in phase with the wave, leading to wave amplification. A more detailed discussion of this mechanism may be found in the Preferred Embodiment section of this specification.
The gyrotron stimulated radiation emission occurs near the frequency .omega.=.OMEGA..sub.c +k.sub.z v.sub.zo. Since .OMEGA..sub.c =eB/.gamma.mc, the radiation wavelength is determined primarily by the strength of the applied magnetic field and is not restricted necessarily by the dimensions of a resonant structure. Thus, unlike most other microwave tubes, the internal dimensions of the device may be large compared to the wavelength, and high power handling capability becomes compatible with operation at millimeter and submillimeter wavelengths. However, it can be seen that some form of control of the magnetic field profile along the fast wave structure is critical to the achievement of any type of wide-band operation. In this regard it has been found that even small changes in the magnetic field at various points along the waveguide interaction region have a great impact on both the efficiency and the gain of the device. In view of the sensitivity to the magnetic field profile, precise control thereof becomes imperative for proper wide-band operation.